Let me paint a picture....
A friend of mine played a long time ago in open beta. He reached Omaha, but returned to Tanks after playing 100 games in Ships.
If he now starts playing and gets lucky, he'll have a Richelieu without much effort. It's a free ship, so of course he's going to play it. We all try out our new toys.
He will get his [edited]kicked because he won't know what's happening. We all know T8 ships see T10 all the time. He never faced a Yamato, he might not even know it exists. He will get deleted by glorious overmatch mechanic. Much fun.
Next game, he will see a Kurfürst and will burn to death by secondaries which he didn't know were a thing. Much fun.
Next game, he will meet a Gearing and be torped to death since he's not known to the clumsyness of T8 BBs. Much fun.
Next game, he has learned and it kemping bush behind an island. The Des Moines on the other side radars him and he gets nuked by a Montana. Much fun.
Next game, he decides to fall back to not get deleted. But he fell back a little too far and is 7 km from the nearest ship. Taiho sees this and deletes him. Much fun.
This is not a far fetched scenario. And that friend of mine is actually better than the average new player since he has Tanks experience, and as such should have some awareness about general game mechanics.
Let's imagine a 50 year old guy (no offence to 50 year old guys here!) that has just started playing. He has never played video games before, but is interested in ships. He has slightly below average hand-eye coordination (as can be expected from a senior, again no offence!). He likes nimble little ships, so his line of choice is IJN DD. He played a few games in T5 at the right time, got lucky with the loot box and gets a Richelieu. After playing some more, he completes the mission and actually gets the ship. He then takes it for a spin without even knowing how to select AP. He shows just a bit too much side and gets citadelled.
This is why this is a bad idea. People should learn by experience. There's progression over the tiers. The gameplay of a T8 is hugely different from T5. Remember you can even earn the ships by playing nothing but Coop.
You cannot throw a 70 battles player into T8 and expect him to do well. He will die a horrible, agonizing death. This is not fun for himself, for his team and even for the enemy.
After a few games of instakill he might even decide this game is not for him and quit forever. Good job WG, you just chased away a potential player and customer.
He might have learned in his own pace by progressing the tiers. He migt have become a decent player. He might have bought some premium ships and time. He might have thrown money at WG because he likes the game. But none of that will happen because he was chased away from the game by WG marketing that decided it's a good idea to let complete newbies be clubbed by unicums in T10s.
I just remembered something that's even worse.... you can compete in Clan Battles with a rental ship if you've played 1 game in T8+
Now let's go back to our 70 battle newbie with his Richelieu. He plays 1 game in the Richelieu and unlocks the rental ships. He can then play Clan Battles.
I cannot stress this enough.... this hypothetical guy will be able to play his 72nd game EVER in a T10 Clan Battle!
And you can easily reach T5 in way less than 70 games, so I'm actually being generous with my assumptions here
Thanks @dasCKD for recommending my post, I'm flattered
Whilst having an easier time getting tier Xs for veteran captains with a whole fleet of tier Xs already is nice and convenient, it is not good for the game for newer players. It may well ruin games for them as they're minnows being let in with the sharks so to speak.